Film press and method for operating a film press

ABSTRACT

A film press for finishing a fibrous web, in particular a paper or board web, includes a roll and a backing roll forming a nip therebetween, and at least one metering rod for contacting at least one of the rolls for applying a liquid or pasty medium thereto to transfer the medium to the fibrous web. The rolls each have a roll core and a roll cover surrounding the roll core. A sensor system having at least one glass fiber into which Bragg gratings are inscribed is disposed in the roll cover and/or between the roll core and roll cover of at least one of the rolls. The sensor system determines a web separation point of the fibrous web from at least one of the rolls after passage through the nip and transfer of the application medium. The web separation point can be controlled by using determined measured values.

The invention is based on a film press for applying a liquid or pasty medium to a fibrous web such as a paper or board web according to the preamble of claim 1 and on a method for operating such a film press according to the preamble of claim 7.

The film press is used in paper machines to apply starch and pigments. The usual technique has been known for a long time.

From a nozzle, liquid medium—coating color or starch suspension—is sprayed onto the application roll and brought to a defined thickness by means of a rotating metering rod. The film thus produced is transferred to the moving paper web in a pressure-loaded nip comprising application roll and backing roll.

It is in the interest of the papermaker to achieve a transfer of the medium which is as complete and uniform as possible, in order to obtain a uniform surface quality. The quality of the distribution depends on many factors, which include, for example, the inclination of the rolls in relation to each other, the elasticity of the roll covers, the paper thickness, properties of the coating medium, working speed, etc.

Here, the pressure distribution in the nip assumes a prominent position. This can generally be determined only roughly from calculations or estimates of boundary conditions, which does not permit satisfactory monitoring.

In what is known as sheet stealing, which occurs during the simultaneous application of medium to both sides of the paper web, there is additionally a prolongation of the contact time between roll and paper web, with the problem of non-uniform liquid application, paper two-sidedness and misting.

It is thus in the interest of the papermaker to obtain information about the pressure distribution in the nip in the film press in order to be able to combat the aforementioned problems.

It has already been known for a long time to equip rolls with sensors in order to obtain information about the operating parameters in a stationary or running machine. The type and number of sensors is very different and reaches from impressions with films with a pressure-sensitive coating as far as highly developed sensor systems which, by using fiber optic sensors with evaluation units and evaluation algorithms, are able to determine and display a high level of information.

Such a fiber optic sensor system, which operates with a fiber optic sensor with inscribed Bragg gratings, is known from WO2010/034321 A1, for example. Here, the Bragg gratings are inscribed into the glass fiber at different or equal intervals and are designed in such a way that they reflect the injected light of different frequencies, the frequencies of which are shifted to a different intensity under the action of pressure, which permits conclusions to be drawn about the pressure distribution in the respective sensor position on the roll. A suitably equipped trigger system permits the assignment to a specific location on the surface of the roll cover.

Hitherto, such sensor systems have primarily been used in paper machines in the press section or during winding. The situation in a film press is considerably more complex, since it is necessary to take account of further components such as the metering rod, which likewise have effects on the operating state of the film press.

It is thus an object of the invention to specify a roll with sensor system which permits the pressure signal, the separation points, the effective length of the nip and the change thereof during continuous operation to be determined and used in a process-effective manner.

According to the invention, the object is achieved by a film press having the characterizing features of the main claim in conjunction with the generic features.

Here, provision is made that, by means of the sensor system, a web separation point of the fibrous web from the roll and/or the backing roll after the passage through the nip and the transfer of the application medium can be determined, and the web separation point can be controlled by using the measured values determined.

The knowledge of this variable in continuous operation can accordingly be used to control the application quantity and to set rheological properties. Paper two-sidedness and grade change can thus be managed better. In addition, it is possible to react more quickly and in an accurately targeted manner to different operating conditions.

Since the signal from the sensors is triggered at the frequency of revolution of the film press roll, monitoring and setting the pressure in the nip are also possible. This has the advantage that, as a result of the monitoring of the pressure profile in the nip, the pressure in the nip and therefore also the penetration depth can be monitored. The calibration with water as application medium makes it possible to considerably reduce the number of parameters that act.

Further advantageous developments and configuration variants of the invention are specified in the sub-claims.

Advantageously, the at least one glass fiber can be arranged on the roll and/or the backing roll in a straight, curved, meandering or spiral manner.

According to an advantageous design variant, the Bragg gratings can be formed at regular or irregular intervals in relation to one another. It is thus possible to take account of any desired roll geometries, for example with cambering or in different hardness regions.

Advantageously, an evaluation unit, by means of which the measured signals from the sensor system (9) can be detected and evaluated, can be provided.

The evaluation unit can be connected to the sensor system.

According to an advantageous aspect of the invention, the evaluation unit can be connected to an open-loop and closed-loop control unit via a cable connection or in a manner communicating without cables. As a result, simple adaptation of the whole of the sensor system to the conditions prevailing in the respective roll position is possible.

In order to control the web separation point from the roll or the backing roll, according to the invention the pressure pulse of the fibrous web leaving the roll or the backing roll can be used as a signal.

The invention will be explained in more detail below with reference to the drawings, without restricting the generality. In the figures:

FIG. 1 shows a schematic lateral illustration of the operating conditions prevailing in the film press of the type mentioned at the beginning,

FIG. 2 shows a graphical illustration of the pressure variation in the nip in a film press,

FIG. 3 shows a schematic illustration of a roll constructed in accordance with the invention for a film press.

FIG. 1 illustrates a highly schematic view of a film press in its essential components. A roll 1 is provided, which interacts with a backing roll 2 to form a nip 3. The fibrous web, in particular a paper or board web 4, is led through the nip 3.

The roll 1 and the backing roll 2 in the exemplary embodiment are each covered by a doctor 5, 6, in a manner that is known and therefore not specifically illustrated, with a film of a liquid or pasty coating medium, which is transferred to the paper web 4 during the passage of the latter through the nip 3. Furthermore, film presses for single-sided coating are also known, in which the backing roll 2 is accordingly not provided with medium.

The transfer of the film from the roll 1 or from the roll 1 and the backing roll 2 is carried out as a result of contact of the liquid film with the paper web 4. The intention is a complete and uniform transfer of the medium over the entire width of the nip 3.

The transfer of the medium begins during the first contact in the positions A and C, which contact is characterized by a pressure rise on the roll surface, and ends with a corresponding pressure drop on the roll surface at the positions B and D. It can easily be understood that here these are not point-like but line-like positions, which appear to be correspondingly point-like because of the non-perspective illustration in FIG. 1.

The quality of the distribution of the liquid film depends on the chronological sequence and the intensity of the pressure distribution during the transfer.

The distance covered as “effective length” in the nip 3 is influenced by the geometric, dynamic and rheological properties of all the units and substances involved at the time of the application. These include, for example, the inclination of the roll 1 and the backing roll 2 in relation to each other, the elasticity of the roll covers 7, 8, described in more detail below, of the roll 1 and of the backing roll 2, the flow behavior and the elasticity of the liquid or pasty medium, the web speed, thickness and quality of the paper web 4 and the speed of the roll 1 and the backing roll 2.

The pressure distribution currently prevailing over the length and width of the nip 3 at the time of the transfer is largely unknown at present, as already explained above. Statements relating to pressure profiles in the nip 3 are based on idealized calculations and on the point force absorption from the roll drives. The exact separation point of the paper web 4 from the roll surface after the passage through the nip 3 can be observed only by means of high-speed photography.

Given simultaneous application of liquid to the top side and the underside of the paper web 4, as a result of the paper web 4 running along on the roll 1 or on the backing roll 2, different separation points from the roll surface occur, which is designated “sheet stealing”. As can be seen in FIG. 1, the separation zone wanders on the roll 1 from position D to position E.

As a result of the sheet stealing, on one side, in this case on the side of the paper web 4 facing the roll 1, prolongation of the contact time between application medium and paper web 4 occurs. As a result of the applied web tension, furthermore an excessively low pressure is exerted on the roll surface by the paper web 4 and the liquid medium. The effective length of the nip 3 is accordingly lengthened on this side as compared with the already contactless paper side.

The asymmetry in liquid application arising from this can be observed as paper two-sidedness on the finished paper surfaces. It is a result of a longer action time (pressure penetration, pressure dewatering) of the application medium on the affected side. Furthermore, the dewatering progress not infrequently acts in the form of undesired, intensified misting during separation of the paper web from the affected side. Compensating for the paper two-sidedness and minimizing the misting usually require of the papermaker asymmetrical intervention in the application formulation and application quantity for the two paper sides and, alternatively or else additionally, by changing the roll positions, in particular by displacing the backing roll 2. For the purpose of avoiding or reducing misting, the web separation point should additionally be located as closely as possible to the exit from the nip 3.

The pressure distribution in the nip 3 and at the metering rods 5, 6 of the film press is critical for the application of the coating color or the starch solution. With a constant speed of the film press and with a constant metering gap, the pressure profile that is established should depend only on the viscosity of the medium (coating color or starch solution). The advantage of controlling the doctor contact pressure via the pressure profile of the roll 1 and the backing roll 2 and not via measuring the contact pressure is that, instead of the pressure, the pressure pulse can be used as a signal. More accurate and quicker metering is therefore possible. Likewise, instead of the pure open-loop control of the contact pressure of the metering rods 5, 6, closed-loop control is now possible.

If the pressure profile in the nip 3 changes, for example as a result of changed penetration of the paper or a different consistency (density, solids content . . . ) of the application medium, this is used as a control signal for readjusting the rolls. If the paper takes up less coating color or starch with a given width of the nip 3 and a given speed, this leads to an increase in the pressure in the nip 3. Here, too, it is possible to build up an open-loop and closed-loop control loop, since set points can be predefined and compared with measured actual values.

In FIG. 2, the sheet stealing situation described above is illustrated graphically. It can be seen that, in the case of the roll 1, the separation zone is displaced and the pressure drop after the passage through the nip takes place with a delay over a relatively long area.

FIG. 3 illustrates, by way of example, a roll 1 or a backing roll 2 which is suitable for use in a film press at the positions described above with doctors 5, 6 and, in a roll cover 7, 8, has a sensor system 9 which picks up the pressure and tension parameters. The sensor system 9 can be provided either in one of the two components 1, 2 or in both components 1, 2.

Here, the roll 1 or backing roll 2 has at least one glass fiber 10, which is arranged in the roll cover 7, 8 or at an interface between roll cover 7, 8 and roll core. Inscribed in the glass fiber 10 are Bragg gratings 11, which can be distributed at regular or irregular intervals in the glass fiber 10. The Bragg gratings 11 can be arranged to be distributed regularly or irregularly in any desired patterns over the surface of the roll 1 or backing roll 2, wherein the at least one glass fiber 10 can be laid in a substantially straight, meandering form, in spirals or other suitable shapes.

The measuring principle is substantially known and will therefore be outlined only briefly, more for completeness. If light with a certain wavelength distribution is injected into the at least one glass fiber 10, said light is reflected at the Bragg gratings 11. The Bragg gratings 11 can be matched to any desired wavelengths as they are inscribed into the glass fiber 10. If, during the passage through the nip 3, the glass fiber 10 is deformed in the area of the Bragg gratings 11, the wavelength of the light reflected at the respective Bragg grating 11 is displaced. The extent of the displacement can be understood as a measure of the deformation and therefore as a measure of the forces acting in the nip 3.

To evaluate the measured signals, it is possible to provide an evaluation unit, which is accommodated suitably on one of the rolls 1, 2 or in the vicinity thereof and is connected to the sensor system 9. The evaluation unit can be connected to an open-loop and closed-loop control unit without cables or in a manner communicating via cables.

Just as the action of a pressure or a local pressure increase can be detected by measurement, the decrease in the pressure can also be measured. As a result of the fact that the sensitivity of the measuring system 9 is very high, it is thus possible, amongst other things, for the web separation point D/E or B in FIG. 2 to be detected by means of suitably equipped roll 1 or backing roll 2. This permits the open-loop and closed-loop control of the roll position, the contact force of the rolls 1, 2 on each other and the contact force of the metering rod 5, 6. 

1-7 (canceled)
 8. . A film press for finishing a fibrous web or a paper or board web, the film press comprising: a roll and a backing roll forming a nip therebetween, said roll and said backing roll each having a respective roll core and a respective roll cover surrounding said respective roll core; at least one metering rod to be brought into contact with at least one of said rolls for applying a liquid or pasty medium to at least one of said rolls in order to transfer the medium to the fibrous web; and a sensor system having at least one glass fiber with Bragg gratings inscribed therein, said sensor system being disposed at least in said roll cover of at least one of said rolls or between said roll core and roll cover of at least one of said rolls; said sensor system being configured to determine a web separation point of the fibrous web from at least one said rolls after a passage through said nip and a transfer of the applied medium and being configured to determine measured values, permitting the web separation point to be controlled by using the determined measured values.
 9. The film press according to claim 8, wherein said at least one glass fiber is disposed on at least one of said rolls in a straight, curved, meandering or spiral manner.
 10. The film press according to claim 8, wherein said Bragg gratings are formed at regular or irregular intervals relative to one another.
 11. The film press according to claim 8, which further comprises an evaluation unit configured to detect and evaluate measured signals from said sensor system.
 12. The film press according to claim 11, wherein said evaluation unit is connected to said sensor system.
 13. The film press according to claim 12, which further comprises an open-loop and closed-loop control unit communicating with said evaluation unit through a cable connection or without cables.
 14. A method for operating a film press for finishing a fibrous web or a paper or board web, the method comprising the following steps: providing a film press including: a roll and a backing roll forming a nip therebetween, the roll and the backing roll each having a respective roll core and a respective roll cover surrounding the respective roll core; at least one metering rod to be brought into contact with at least one of the rolls for applying a liquid or pasty medium to at least one of the rolls in order to transfer the medium to the fibrous web; and a sensor system having at least one glass fiber with Bragg gratings inscribed therein, the sensor system being disposed at least in the roll cover of at least one of the rolls or between the roll core and roll cover of at least one of the rolls; the sensor system being configured to determine a web separation point of the fibrous web from at least one the rolls after a passage through the nip and a transfer of the applied medium and being configured to determine measured values, permitting the web separation point to be controlled by using the determined measured values; and using a pressure pulse of the fibrous web leaving the roll or the backing roll as a signal in order to control the web separation point from the roll or the backing roll. 